Lithium secondary batteries are recently in the spotlight as a driving power source for electronic devices. As a negative electrode material for the lithium secondary battery, graphite has been mainly used. However, the graphite has a low capacity per unit mass of 372 mAh/g. Thus, preparation of a high-capacity lithium secondary battery is difficult.
As a negative electrode material exhibiting higher capacity than graphite, there are materials forming an intermetallic compound with lithium, such as silicon, tin, an oxide thereof, and the like.
However, volumes of the above materials are expanded because crystal structures are changed when absorbing and storing lithium. When silicon absorbs and stores the maximum amount of lithium, the silicon is transformed into Li4.4Si and the volume is expanded by a charging. The rate of increase in volume caused by the charging is expanded up to about 4.12 times than the volume of the silicon before the volume expansion. For reference, the volume expansion ratio of the graphite currently used for the negative electrode material is about 1.2 times.
Therefore, a significant amount of research on a high capacity of the negative electrode active material such as the silicon, that is, a research for decreasing the volume expansion ratio by alloying of silicon has been performed. However, the research is not practical because a metal such as Si, Sn, Al, and the like is alloyed with the lithium during charge and discharge, as a result, the volume expansion and contractions occur. Thus, the metal is micronized and the cycle characteristics of the battery are degraded.
Silicon is known as an element most likely to have the high capacity. However, the amorphization of the silicon by itself alone is difficult. The amorphization of the alloy containing the silicon as a main component is also difficult.
Moreover, the silicon-based negative electrode active material has another problem because whose crystal is highly brittle. Thus, cracks suddenly occur inside the negative electrode active material located in the electrode during repeated intercalation and deintercalation of the lithium, as a result, the life characteristics of the battery are deteriorated immediately.